Volcanic margins

Rifted volcanic margins are defined by the occurrence of the following three components: Large Igneous Provinces (Section 7.4.1) composed of thick flood basalts and silicic volcanic sequences, high velocity (Vp > 7 km s-1) lower crust in the continent-ocean transition zone, and thick sequences of volcanic and sedimentary strata that give rise to seaward-dipping reflectors on seismic reflection profiles (Mutter et al., 1982). The majority of rifted continental margins appear to be volcanic, with some notable exceptions represented by the margins of the Goban Spur, western Iberia, eastern China, South Australia, and the Newfoundland Basin-Labrador Sea. Relationships evident in the Red Sea and southern Greenland suggest that a continuum probably exists between volcanic and nonvolcanic margins.

The high velocity lower crust at volcanic margins occurs between stretched continental crust and normal thickness oceanic crust (Figs 7.31, 7.32). Although these layers have never been sampled directly, the high Pn wave velocities suggest that they are composed of thick accumulations of gabbro that intruded the lower crust during continental rifting. The intrusion of this material helps to dissipate the thermal anomaly in the mantle that is associated with continental rifting.

The Lofoten-Vesteralen continental margin off Norway (Figs 7.31, 7.32) illustrates the crustal structure of a volcanic margin that has experienced moderate extension (Tsikalas et al., 2005). The ocean-continent transition zone between the shelf edge and the Lofoten basin is 50-150 km wide, includes an abrupt lateral gradient in crustal thinning, and is covered by layers of volcanic material that display shallow seaward dipping reflectors (Fig. 7.32a). The 50-150 km width of this zone is typical of many rifted margins, although in some cases where there is extreme thinning the zone may be several hundred kilometers wide. Crustal relief in this region is related to faulted blocks that delineate uplifted highs. In the Lofoten example, the continent-ocean boundary occurs landward of magnetic anomaly 24B (53-56 Ma) and normal ocean crust occurs seaward of magnetic anomaly 23 (Fig. 7.31b). Crustal thinning is indicated by variations in Moho depth. The Moho reaches a maximum depth of 26 km beneath the continental shelf and 11-12 km beneath the Lofoten basin. Along profile A-A' a region of 12-16 km thick crust within the ocean-continent transition zone coincides with a body in the lower crust characterized by a high lower crustal velocity (7.2 km s-1) (Fig. 7.32a,c). This body thins to the north along the margin, where it eventually disappears, and thickens to the south, where at one point it has a thickness of 9 km (Fig. 7.31c). Oceanic layers display velocities of 4.5-5.2 km s-1, sediments show velocities of <2.45 km s-1. These seismic velocities combined with gravity models (Fig. 7.32b) provide information on the nature of the material within the margin (Fig. 7.32c).

In most volcanic margins the wedges of seaward-dipping reflectors occur above or seaward of the high velocity lower crust in the continent-ocean transition zone. Direct sampling of these sequences indicates that they are composed of a mixture of volcanic flows, vol-caniclastic deposits, and nonvolcanic sedimentary rock that include both subaerial and submarine types of deposits. Planke et al. (2000) identified six units that are commonly associated with these features (Fig. 7.33): (i) an outer wedge of seaward-dipping reflectors; (ii) an outer high; (iii) an inner wedge of seaward-dipping reflectors; (iv) landward flows; (v) lava deltas; and (vi) inner flows. The wedge-like shape of the reflector packages is interpreted to reflect the infilling of rapidly subsiding basement rock. The outer reflectors tend to be smaller and weaker than the inner variety. The outer high is a mounded, commonly flat-topped feature that may be up to 1.5 km high and 15-20 km wide. In some places this may be a volcano or a pile of erupted basalt. Landward flows are subaerially erupted flood basalts that display little to no sediment layers between the flows. The inner flows are sheet-like bodies located

landward and, typically, below the lava delta. Lava deltas form as flowing basalt spills outward in front of the growing flood basalts. The emplacement of these features is associated with the establishment of thicker than normal ocean crust within the continent to ocean transition zone (Planke et al., 2000).

The conditions and processes that form volcanic rifted margins are the subject of much debate. In general, the formation of the thick igneous crust appears to require larger amounts of mantle melting compared to that which occurs at normal mid-ocean ridges. The origin of this enhanced igneous activity is uncertain but may be related to asthenospheric temperatures that are higher than those found at mid-ocean ridges or to unusually high rates of upwelling mantle material (Nielson & Hopper, 2002, 2004). Both of these mechanisms could occur in association with mantle plumes (Sections 5.5, 12.10), although this hypothesis requires rigorous testing.

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